New solar cell structure sets record

24. September 2013 | Applications & Installations, Markets & Trends, Research & Development | By:  Shamsiah Ali-Oettinger

A new world record efficiency for solar sunlight conversion has been set by a cell structure with four solar sub-cells. The solar cell has managed to hit a new record efficiency of 44.7% and is the brainchild of researchers from the Fraunhofer Institute for Solar Energy Systems ISE, Soitec, CEA-Leti and the Helmholtz Center Berlin.

The four-junction solar cells consist of several cells made out of different III-V semiconductor materials stacked on top of each other.

The German-French team from the four associations had already announced a 43.6% efficiency for their solar cell in May this year. But building on that groundwork, the team of researchers have now managed to boost the efficiency of the solar cell to 44.7%. The efficiency was measured at a concentration of 297 suns. This indicates that 44.7% of the solar spectrum's energy, from ultraviolet through to the infrared, is converted into electrical energy.

The four-junction solar cells are used in concentrated PV or CPV and consist of several cells made out of different III-V semiconductor materials (composed of elements from III-V groups of the periodic table) stacked on top of each other. The single sub-cells absorb different wavelength ranges of the solar spectrum. 

"This four-junction solar cell contains our collected expertise in this area over many years. Besides improved materials and optimization of the structure, a new procedure called wafer bonding plays a central role. With this technology, we are able to connect two semiconductor crystals, which otherwise cannot be grown on top of each other with high crystal quality. In this way we can produce the optimal semiconductor combination to create the highest efficiency solar cells," said Frank Dimroth, Department Head and Project Leader in charge of the development work at Fraunhofer ISE.

Soitec's CEO and Chairman André-Jacques Auberton-Hervé believes that with this record, the four-junction solar cell design based on Soitec's bonding techniques and expertise can potentially contribute to the competitiveness of Soitec's own CPV systems.  Soitec (started in 2005 under the name Concentrix Solar, a spin-off of Fraunhofer ISE) produces CPV systems. Presently Soitec has CPV installations in 18 different countries including Italy, France, South Africa and California.

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